conditions. However, the generator breaker normally exists between the protected
generator and its power transformer.
Neutral
side
x E
3H
R
f
(1-x) E
3H
3*C
g/2
Step-up unit Generator CB Circuit breaker
Terminal
side
Stator winding
3*C
g/2
3*C
t
C
T
= 3*C
g
/2 + 3*C
t
or
C
T
= 3*C
g
/2
C
N
= 3*C
g
/2
Fault at distance ‘x’
from generator neutral
x
(1-x)
Figure 317: Capacitance seen at generator terminal and neutral side
C
g
Phase capacitance of generator stator winding
C
t
Total external phase capacitance of the system as seen from the generator
C
N
Phase-to-ground lumped capacitance of the generator stator winding, be-
tween the earth-fault location and the generator neutral
C
T
Phase-to-ground lumped capacitance of the generator stator winding, be-
tween the generator terminal and earth-fault location
When there is a generator breaker, the capacitive coupling to earth differs between
the operating conditions when the generator is running with the generator breaker
open (before synchronization) and with the circuit breaker closed.
With the generator breaker open, the total capacitance is smaller compared to
normal operating conditions. This means that the neutral side third harmonic
voltage is reduced compared to the normal operating condition. Therefore, there
is a need to reduce the sensitivity of the protection. When generator breaker is
open, H3EFPSEF desensitizes the protection by multiplying the
Beta
setting with a
set constant
CB open factor
setting.
U U U
H T H N H N3 3 3_ _ _
+ ≥ × ×CB open factor Beta
(Equation 86)
The
CB Open factor
setting is obtained during commissioning.
Protection functions
1MRS759142 F
550 REX640
Technical Manual
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